NYC Transit supervisors falsified thousands of vital signal inspections across the subway system for years, leaving straphangers at risk for deadly collisions like the one that killed nine people in Washington, DC, The Post learned.

Across every line, a cabal of managers in the signal department forced maintainers to fib on the inspections by threatening them with punishments like loss of overtime, according to a sweeping six-month investigation by the inspector general of the MTA, which oversees NYC Transit.

At least one high-level chief, Tracy Bowdwin — the MTA’s highest-earning Signal Department supervisor at $165,000 a year — was demoted last week in the fallout, and managers are still being questioned, transit sources said.

The dangerous practice was a response to ramped-up pressure from the MTA to meet federal standards that call for railway switches and signals to be inspected monthly, sources said.

“Instead of five signals to inspect [in a shift], they would give you 15. There’s no way 15 could be done, but they would say you had to do it,” one signal maintainer said. “It’s like you think your car is fine after going to the mechanic, but they never looked at it.”

The more-than-1,000 signal maintainers on the TA’s payroll, who are members of the Transport Workers Union Local 100, make a base salary of about $60,000 a year — with the highest overtime earners pulling in as much as $127,000, according to MTA records. The 100 or so managers, who don’t earn overtime, make up to $165,000 a year.

Signal maintainers would routinely enter false inspections into their logbooks, which managers used to write reports. In some cases, managers would write a bogus report even if a worker refused to enter the fudged data in his book.

Workers who didn’t comply lost overtime privileges or got sent to the dirtiest, most leak-infested tunnels, sources said.

The IG undertook the investigation to follow up on two previous allegations of lax signal inspections. A 2006 report found that “the system lacked internal controls to prevent fraud and falsification of inspection and maintenance records,” although the falsification allegation could not be substantiated. NYC Transit said it had tried to fix the problem.

In 2000, MTA signal maintainers were also found to have falsified inspection reports on some 2,000 signals.

There are thousands of signals throughout the subway system — and a false green light can lead to potentially deadly rear-end collisions. The greatest risk is along the L line, where trains are operated robotically and there are fewer fail-safes, union officials said.

Other lines have stopping mechanisms that trigger the brakes if a signal fails, though a bogus red signal can cause unnecessary delays in service.

A signal malfunction caused Washington’s fatal June 2009 commuter-rail crash, in which a Metro train crashed into the back of another.

Falsification of records is a felony offense, though it was unclear whether the managers and the workers will face criminal charges.

Although the investigation is not yet complete, transit workers have been ordered over the past few months to comb over all signals, said MTA Inspector General Barry Kluger.

“We also took swift action to ensure that none of these deficiencies undermined the signal system’s safe operation or its underlying components,” NYC Transit Spokesman Charles Seaton said. “The signal system is safe.”